Which of these numbers is prime? ${9,\ 38,\ 39,\ 81,\ 97}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 9 are 1, 3, and 9. The factors of 38 are 1, 2, 19, and 38. The factors of 39 are 1, 3, 13, and 39. The factors of 81 are 1, 3, 9, 27, and 81. The factors of 97 are 1 and 97. Thus, 97 is a prime number.